Many measurement instruments utilize heterodyne receivers. The receivers are used to down-convert an input signal to an intermediate frequency (IF) that can then be analyzed to determine the characteristics of the input signal in some frequency band that is much less than the highest frequency in the input signal. FIG. 1 illustrates a typical heterodyne receiver 10.
A repetitive input signal, typically generated by a device under test (DUT) is down-converted by mixing the signal with the signal from a local oscillator (LO) 11 in a mixer 12 to create an IF signal. The IF signal consists of tones that correspond to some of the tones in the input signal. By characterizing the tones in the IF signal, information about the corresponding tones in the input signal is obtained. The output of mixer 12 is digitized in an analog-to-digital converter (ADC) 13 to create a sequence of digital values that are transformed by a fast Fourier transforms (FFT) or similar transform using digital signal processor (DSP) 14 to provide the amplitude and a set of phases corresponding to the various frequency components in the IF signal. The amplitudes of the tones do not depend on the phase of the LO signal or the time at which the sequence of digital values begins relative to the phase of the IF signal.
The calculated phases, in contrast, are determined by the phases of the components in the input signal, the phase of the LO relative to those phases and the time at which the digital sequence is generated. In many situations, the input signal is characterized by down-converting the input signal with a number of different LO frequencies that are generated by controller 15. Hence, the final result will depend on the differences in phases of the LO signals from frequency to frequency. However, even if the phases of the LO signals are known at the LO signal generator for each of the frequencies, the components between the LO signal generator and the mixer alter the phase. Hence, a method for determining the contributions of these components as a function of frequency is required.